The invention relates to an orthodontic appliance for the forward displacement of the lower jaw of a patient, which is usually referred to as Herbst hinge.
Such appliances comprise a pair of telescopes, which each consist of a metal telescopic tube and a metal slide rod guided therein, and which are attached on both sides of the patient""s jaw, for example, via orthodontic bands, to the patient""s upper and lower jaws.
The appliance known in the literature as Herbst hinge can be compared to an artificial joint between upper jaw and lower jaw (H. Pancherz, Kleines Lehrbuch der Angle-Klasse II,1 unter besonderer Berxc3xccksichtigung der Behandlung, Chapter 16, pages 225 to 251, Quintessenz Verlags-GmbH, Berlin). When the jaw is closed the telescopes are retracted or telescoped fully, and the Herbst hinge can thus apply therapeutically desired forward pushing forces to the lower jaw.
When the patient opens his jaw the slide rod travels a certain distance out of the telescopic tube and thus allows the patient limited space for his jaw movements. Owing to the rigidity of the metallic telescopic tubes and slide rods, the Herbst hinge is, however, relatively rigid and results in considerable limitation of the freedom of movement of the lower jaw. If the appliance is inappropriately fitted undesirably large forces can also be exerted on the jaw or the appliance can be even be destroyed as a result of this.
As a rule, the Herbst hinge is designed as a so-called firmly attached fixture in order to permanently exert the necessary forward pushing force on the lower jaw during the treatment. However, it is not possible for the patient to briefly remove the appliance to clean it for oral hygiene purposes.
Different solutions for attaching the telescopes to the upper and lower jaws have been discussed in the literature. For example, the textbook cited above describes the attachment by screws to attachment parts affixed to the molars of upper and lower jaws, as does U.S. Pat. No. 4,462,800. Another solution which works with a pinning together is known from U.S. Pat. No. 5,183,388. Attachment of the telescopic rods to the molars by ball pins and securing rings is recommended in PCT/DK94-00183.
All these solutions have the disadvantage that the attachment elements (e.g. screws, pins, securing rings) naturally have to be very small, which makes it difficult even for experienced staff to insert or remove the Herbst hinge, and in individual cases these small parts, if handled clumsily, can be swallowed by the patient.
The object of the invention is to further develop an appliance of the kind described at the outset so that while the therapeutically desired forward push on the lower jaw remains ensured, a greater freedom of movement is imparted to the lower jaw.
This object is accomplished in accordance with the invention with the appliance described at the outset in that the telescopes in the telescoped state are rigid, as known per se, but in the extended state are flexible.
This constructional principle ensures that in the closed state of the jaw (simultaneously the retracted or telescoped state of the telescopes) the desired forward pushing forces can act, but accord the lower jaw of the patient to be treated a considerably increased freedom of movement when he opens his jaw (forward pushing forces are not active here with the Herbst hinge).
A specific embodiment of this principle consists in that the telescopic tubes are of rigid design, but the telescopic rods are elastically deformable.
As an alternative thereto, in another embodiment the slide rod can also be of rigid design, but the section guided in the telescopic tube includes a conical area. Herein, as soon as the slide rod is moved a certain length out of the telescopic tube, the play between the telescopic tube and the slide rod increases, which increases the freedom of movement of the lower jaw. If, in addition, the slide rods are produced from an elastically deformable material, a particularly good effect is achieved with respect to increasing the freedom of movement of the lower jaw.
The conical area preferably extends over at least one fourth of the length of the telescoping sections of the slide rods. Thus, more freedom of movement is already created for the lower jaw at a relatively early stage of opening the jaw.
An angle of taper of approximately 2xc2x0 is already sufficient for a noticeable increase in the freedom of movement. As a rule, a maximum angle of taper of approximately 15xc2x0 is adequate to accord highly improved wearing comfort to the patient. On the other hand, a limitation of the angle of taper to 15xc2x0 eliminates the danger of the slide rod tilting in the telescopic tube and exposes in the extended state of the telescope only a small cross-sectional area of the mouth of the telescopic tube, through which undesired materials such as, for example, bits of food, etc., can penetrate into the telescope so that this limit is also recommended from the point of view of oral hygiene.
In general, materials whose modulus of elasticity in tension lies in the range of from 100 N/mm2 to 20,000 N/mm2 are suitable for the manufacture of the inventive telescopes.
In accordance with the present invention, plastic is a preferred material. It is not only easy to work to the required shapes, but also offers aesthetic advantages over telescopes made of metal. Both rigid and elastically deformable slide rods can be made of plastic. Plastic is also suitable for the manufacture of telescopic tubes. In the case of the latter, it is recommended, in particular, to use transparent plastic, which not only allows checking of the choice of the correct length of the slide rods, but, in addition, also checking as to the cleanliness and the oral hygienic state of the telescopes, in particular, the interior of the telescopic tube. Preferred elastic plastic materials are those of the polyolefins, EVAs and polyamides. These plastic materials have a modulus of elasticity in tension in the range of approximately 500 to 2,500 N/mm2 (German Industrial Standard 53457) and do not exhibit any breakage in the impact test (German Industrial Standard 53453). Moreover, these plastic materials allow an elastic deflection of the slide rods and/or telescopic tubes without any noticeable deformation remaining after the deflection.
Ball joints are also preferably used in the present invention for attaching the telescopes to upper and lower jaws. Here a ball head attached to the tooth serves for support on the jaw, while a ball socket formed in each case at one end of slide rod and telescopic tube engages over this ball head. The ball socket is preferably of elastic resilient design so that it is snappable onto the associated ball head.
The snapping of the ball socket onto the associated ball head is facilitated by the choice of a sufficiently elastic plastic material. However, limits are set here as the ball socket must be inherently stable enough for it to transmit the correcting forces unadulterated onto the ball head.
Alternatively, the ball socket can be provided with one or several radially extending slots which extend as far as the mouth opening of the socket. In this case, a stiffer plastic material can then be used.
Finally, it is to be ensured that the holding forces between ball head and ball socket are large enough for unintentional detachment of the snap connection to be avoided.
Again the use of plastic for the fabrication of the slide rods and the telescopic tubes, which, in this case, are integrally formed with the ball sockets is favorable for this construction. However, the use of metals which are generally recognized as safe from the point of view of oral hygiene, such as, for example, nickel-free steels or titanium, is recommended for the ball heads. Not only do metallic ball heads better resist the forward pushing forces which they are to transmit onto the lower jaw, but they are also easier to attach with the necessary security, for example, by means of metallic tooth bands, to the patient""s teeth.
A continuous opening in the interior of the telescopic tubes, which leads to the cavity of the ball socket, serves to facilitate cleaning of the telescopes. Foreign matter which has unintentionally penetrated into the telescopic tube can thus also be easily flushed out by the patient himself, and the telescopes can be kept in perfect condition as regards oral hygiene by the patient himself. Here it is, of course, helpful for the telescopic tubes, as already mentioned further above, to be made of transparent plastic.
Provision is made in a preferred embodiment for the telescopic tubes to have in their interior a stop for maximum slide-in of the slide rods. When the telescope is retracted, which results in transmission of the forward pushing forces, the corresponding end of the slide rod is supported on the stop, and the slide rod is thus prevented from being pushed in too far, which could result in jamming of the free end of the slide rod. Moreover, this also ensures that the therapeutically desired forward pushing forces are maintained when the telescope is in the retracted or telescoped state.
In addition, provision may be made for spacers forming the stop to be inserted into the interior of the telescopic tube so as to alter the forward pushing forces during the patient""s treatment, or to adapt the length of the telescope in the retracted or telescoped state to the treatment success which has already occurred. Alternatively, this can, of course, also be done by adapting or exchanging slide rods of a different length.
As known per se, the transmission of forces can also take place via a stop which is arranged at the end of the telescopic slide rod adjacent to the ball socket. This stop is then supported at the free end of the telescopic tube.